There's at least 6,892 unique accounts. I ran a few searches across all players that were in a clan (as of 4 days ago) or had ever competed in a ladder (seasonal, RT, 1v1, 2v2, or 3v3) and that's the number of accounts I found- including inactives and all.

If you want to find all accounts (no real way to exclude alts, but you can exclude inactive accounts, accounts that have played 0 games, accounts that never finished any of the single-player levels, etc.), you'll basically have to search the entire possible range of IDs (which I believe to be from 10000000 to 10000000000*- so that's 9,990,000,000 (successful) requests to the Warlight server, and 9,990,000,000 pages to scrape and parse unless there's a better way or Fizzer just hands you the answer). This is where BeautifulSoup comes in handy.

* from what I can tell, ID's range from 8 digits to 10 digits in length and never begin with a 0, so they cover at most the integer range [10000000, 10000000000).